Toluene hydrogenation was studied within a wide range of hydrogen sulfide, hydrogen, toluene, and ammonia partial pressures on a standard sulfided NiMo/Al2O3 hydrotreating catalyst at 350 degreesC under a total pressure of 4.9-9.8 MPa. The results showed a complex inhibiting effect of H2S on the hydrogenation activity with an order of reaction relative to H2S varying between -0.05 and -0.5. Unexpectedly, this inhibiting effect was enhanced by the presence of ammonia. Several kinetic models based on the homolytic or heterolytic dissociation of hydrogen and hydrogen sulfide were investigated and discriminated by using the CHEMKIN/SURFACE CHEMKIN II tool. The heterolytic dissociation was supposed to occur on centers composed of an unsaturated Mo ion (on which hydride ions and organic molecules can adsorb) and of a sulfur anion, host of the proton generated by the heterolytic dissociation of hydrogen or hydrogen sulfide. It was concluded that toluene hydrogenation was most likely to occur through a heterolytic mechanism starting with a hydride addition followed by a proton addition. In the range of H2S partial pressures investigated, the latter was the rate-determining step.